"...the behaviour of most present day humans remains moderated by magical thinking-type mental processes (lack of integration between the left prefrontal cortical areas and memory), underwritten by sub-optimal cause and effect perception."

Robert G. Bednarik, An aetiology of hominin behaviour, Homo, 2012

Wednesday, 15 April 2015

Not by the hair on your chinny chin chin

New research reported in the Journal of Anatomy from the University of Iowa reaches an unsurprising conclusion: that the development of the chin did not result from mechanical forces such as chewing.

In fact, the prominence of the chin in present day humans is well documented and understood.

"Compared to chimpanzees and macaques, human skulls exhibit a derived spatial distribution of growth fields, especially in the face (Enlow, 1990). While the internal (basicranial) surface of the maxilla grows via interosseous bone deposition (sutural bone growth), the external (facial) surface of the maxilla as well as the external symphyseal area of the mandible exhibit resporptive fields. As an effect, the human face grows inferiorly and anteriorly through maxillobasiocranial bone apposition, but forward growth is counteracted by maxillofacial resportive growth fields. The combination of these processes results in a retracted, vertically oriented face in which the chin represents the most prominent (i.e., least resorbed) part."

Zollikofer C., 2012, Evolution of hominin cranial ontogeny, Progress in Brain Research, 195: 273-289.

Clearly the chin is a neotenous feature but Robert Franciscus from the University of Iowa proposes that it is evidence of so-called "modern humans" replacing "Neanderthals" around 60,000 years ago. Past horizons ( quote him saying:

"What we’re arguing is that modern humans had an advantage at some point to have a well-connected social network, they can exchange information, and mates, more readily, there’s innovation and for that to happen, males have to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture.”

So in a desperate bid to support the contention that "modern humans" outcompeted or absorbed Neanderthals Franciscus turns incredibly to face architecture. Whilst it is probably correct to imply that the transition from the robust hominins of the past to the gracile hominins of the present day may be in part explained by changing hormone levels (as a result of increasingly culturally mediated mating behaviour for neotenous features) there is no evidence to support the contention that the chin is a feature found only in populations exiting Africa, or indeed that there were well connected social networks exclusive to these groups.

Scott elaborated on her research at the AAPA 2015 conference which refutes the suggestion that the prominent chin observed in extant humans is an autapomorphy upon which to differentiate between "modern humans" and "Neanderthals".
"...this research, demonstrating overlap in overall anterior symphyseal shape between H. sapiens and Neandertals, raises questions about the distinctiveness of the human chin."

As noted on numerous occasions on this blog there is no evidence of a "replacement" of Neanderthals in Europe by "modern humans" let alone evidence for the "well connected social networks" that Franciscus refers to. Past Horizons have confused matters further by referring to work that does not support an evidence base for morphological distinction between so called species but rather underlines the key learning: extant autapomorphies are a result of pedomorphosis through neoteny, a heterochronic process.